You need to attend the school functions. If you do not attend, then you really cannot complain about decisions not being made in your favor.
Explain to the teacher that you have custody, the girlfriend is not party to this whatsoever. The teacher should understand and respect your wishes since you are the custodial parent.
You might want to bring along the decree to show you have custody.
Don't let them intimidate you. If the girlfriend speaks, remind her that this meeting with the childs parents and that you would appreciate it if this meeting was with the father and you only. If she wants to sit in and listen then she can, but she needs to close her mouth.
As long as the father is there giving her permission to sit in, the school probably won't do much. You do have a voice though and you have more rights then her, don't let her call the shots.
If he glares at you, pay no attention. Limit eye contact except for if he wants to ask you a question.